1. Field of the Invention
The present invention relates to a lithium rechargeable battery adapted to more easily evacuate gas generated inside the battery and improve the stability of the battery.
2. Discussion of the Background
As portable electronic appliances continue to be made lighter and more compact, small-sized high-capacity batteries have become increasingly necessary as a power source. Lithium rechargeable batteries are increasingly used in the industry because they have a high energy density per unit weight and an operating voltage of 3.6V, which is three times larger than that of nickel-hydrogen or nickel-cadmium batteries.
Lithium rechargeable batteries create electric energy by oxidation and reduction reactions that occur during intercalation and deintercalation of lithium ions at the positive and negative electrodes. Materials enabling lithium ions to undergo reversible intercalation and deintercalation are used as the active materials of the positive and negative electrodes. An organic electrolyte or a polymer electrolyte is used to fill the space between the positive and negative electrodes.
Lithium-containing metal oxide may be used as the positive electrode active material of the lithium rechargeable batteries. Examples of a lithium-containing metal oxide include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium manganese oxide (LiMnO2).
Lithium or lithium alloy is conventionally used as the negative electrode active material. Lithium has the drawback that the batteries tend to short-circuit and explode due to dendrite formation. To overcome this problem, lithium has been replaced by carbon-based materials, including amorphous and crystalline carbon. The lithium rechargeable batteries are manufactured in various shapes including cylinders, squares, and pouch types.
FIG. 1 is an exploded perspective view showing a conventional lithium rechargeable battery.
Referring to FIG. 1, a lithium rechargeable battery is formed by placing an electrode assembly 12 including a first electrode 13, a second electrode 15, a separator 14, and an electrolyte into a can 10 and sealing the opening of the can 10 with a cap assembly 70.
The cap assembly 70 includes a cap plate 71, an insulation plate 72, a terminal plate 73, and an electrode terminal 74. The cap assembly 70 is coupled to the opening of the can 10 and to a separate insulation case 79 that seals the can 10.
The cap plate 71 is made of a metal plate with a size and a shape corresponding to the shape of the opening of the can 10. The cap plate 71 has a terminal through-hole of predetermined size formed at its center, into which the electrode terminal 74 is inserted. A tubular gasket 75 is coupled to the outer surface of the electrode terminal 74 to insulate the electrode terminal 74 from the cap plate 71. The cap plate 71 has an electrolyte injection hole 76 of predetermined size formed on a side. The cap assembly 70 is connected to the top opening of the can 10. An electrolyte is injected via the electrolyte injection hole 76, and the electrolyte injection hole 76 is then sealed by a plug 77.
The electrode terminal 74 is electrically connected to a second electrode tab 17 of the second electrode 15 or to a first electrode tab 16 of the first electrode 13 via the terminal plate 73, which acts as a second or first electrode terminal. Insulation tapes 18 are wound around portions of the electrode assembly 12 through which the first electrode tab 16 and the second electrode tab 17 are drawn to avoid a short circuit between the electrodes 13 and 15. The first electrode 13 or the second electrode 15 may act as either a positive electrode or a negative electrode.
A conventional lithium rechargeable battery is in danger of fracture if the voltage abruptly rises due to an internal or external short circuit or overcharging or over-discharging of the electrode assembly. When the battery is overcharged, excessive deintercalation of lithium occurs at the positive electrode and excessive intercalation of lithium occurs at the negative electrode. This renders the positive and negative electrodes thermally unstable and generates radical heating reactions, including decomposition of the organic solvent of the electrolyte, reaction between the negative electrode active material and the electrolyte, and solid electrolyte interface (SEI) film thermal decomposition reaction of the negative electrode. In addition, a thermal runaway phenomenon occurs and seriously degrades the stability of the battery.
The lithium rechargeable battery is equipped with a safety device, such as a positive temperature coefficient (PTC) thermistor or a safety vent to prevent the battery from catching fire or exploding due to an abnormality. The safety vent is formed on the cap plate or on the can and is adapted to open at a predetermined pressure to evacuate gas inside the battery to the exterior.
Despite being equipped with such a safety device, the danger remains that the safety device may fail to function in time to avoid a fracture if gas is not properly evacuated from the electrode assembly.